In recent years, composites have found many applications in structural parts. Their practical use in precision assemblies, however, requires post-manufacturing, or secondary operations. Among these operations are drilling, machining, grinding, lapping, and other material-removing processes.
Generally speaking, it is difficult to perform these material-removing processes with composites. The high hardness characteristic of such materials frequently requires expensive diamond tooling. In addition, machining away layers of a composite will frequently remove needed structural integrity.
For this reason, there is a need for a sacrificial interface layer of material not needed for its structural characteristics. Moreover, it is recognized that there is a need for being able to conveniently and economically repair or rework a surface of a composite even after it has been machined since such materials conventionally require elaborate surface preparation, extraordinary measures to apply pressure, reheating and extremely careful lay-up to restore a worn or damaged surface. Even with such surface preparation and extraordinary measures, contamination can make repair or rework procedures unreliable at best.
Among the efforts to provide a metal matrix composite is that disclosed in Elban et al U.S. Pat. No. 3,833,402, issued Sept. 3, 1974. Elban et al disclose a graphite fiber treatment comprised of depositing a metal-containing compound on a graphite fiber and decomposing the metal containing compound at elevated temperatures in an inert atmosphere. However, while providing a matrix for a composite, there is no suggestion of providing a composite having a machineable surface.
A similar attempt to provide carbon fiber-reinforced metal is disclosed in Beutler et al U.S. Pat. No. 3,720,257, issued Mar. 13, 1973. Beutler et al discloses a method in which carbon fibers are coated with nickel, the coated fibers are combined with metal heated in a crucible under a vacuum, and the coated fibers and heated metal are allowed to solidify. Once again, though, Beutler et al fail to suggest a composite having a machineable surface.
Levinstein et al U.S. Pat. No. 4,023,252, issued May 17, 1977, is directed to a mixture of a plurality of powdered materials in specific percentages. The mixture includes an aluminum-copper solid solution alloy powder and a nickel-graphite powder. With these powdered materials, Levinstein et al provide a wear layer consisting of an abradable coating rather than a composite having a machineable surface.
Similarly, Cowden et al U.S. Pat. No. 3,084,064, issued Apr. 2, 1963, is directed to an abradable metal coating in the form of a wear layer. The coating is applied directly to a base material and flakes off in powder form when subjected to high forces along its surface. As a result, Cowden et al fail to suggest a composite having a surface machineable to a precision dimension but depends instead on the "wearing in" of components through abrasion.
The present invention is directed to overcoming the above stated problems and accomplishing the stated objects.